Offshore platforms, used for oil and natural gas production and a number of other utilities are often susceptible to wind, wave, and current forces. As a result, such platforms must overcome such forces in order to maintain a relatively fixed position. It is important that the floating structure remain stable throughout its operation so that it is safe for people to work on the structure and so that the structure can function properly. Traditional catenary moorings typically consist of freely hanging lines that connect a floating structure to anchors, or piles, on the seabed, positioned at some distance from the floating structure. Steel-linked chain and wire rope have conventionally been used. These lines form a catenary shape and rely on an increase or decrease in line tension as the lines lift off or settle on the seafloor. This is needed to produce a restoring force as the surface platform is displaced by the environment. Because the restoring force provided by the traditional mooring system is generally small, the floating platform experiences all six degree of freedom motions.
Another type of mooring system uses tendons and is typically used in Tension-Leg Platforms (TLP). The mooring system is made up of a set of tension legs, or tendons, which attach to the platform and connect to a template or foundation on the seafloor. The foundation is held in place by piles either driven or sucked into the seafloor. This method dampens the vertical and rotational motions of the platform, but allows for horizontal movements. Tendons are typically steel tubes with dimensions of 2-3 foot in diameter with up to 3 inches of wall thickness, and lengths which depend on the water depth. Because of the excessive loads from wave, wind and current actions, tubular tendon components have to be manufactured by special technique with high cost materials such as titanium.
The two existing mooring systems fail to completely limit the movements of a floating platform and do so in a cost-effective manner. Accordingly, there is a present need for simple and inexpensive method for effectively anchoring a floating platform to the seabed. The present invention uses tendons which may be made from one or more wire cables or chains to tether the columns of a platform to the seabed. Each of the tendons are tensioned between the column and the anchor to limit motions of the platform in all directions.